1. Field of the Invention
The present invention relates to a steering device.
2. Description of the Related Art
An electric power steering device composed of a ball screw mechanism attached to the outer periphery of a rack shaft and a motor coupled to the ball screw mechanism via a speed reduction mechanism is provided as one type of steering devices. In the electric power steering device, when rotation of an output shaft of the motor is transmitted to the ball screw mechanism via the speed reduction mechanism, the ball screw mechanism applies a force in the axial direction to the rack shaft. The force in the axial direction applied to the rack shaft serves as an assist force to assist a driver in performing a steering operation. A support structure for a ball screw mechanism in an electric power steering device of this type according to the related art is described in International Publication No. 2011/147824 (WO 2011/147824).
In the electric power steering device described in WO 2011/147824, as illustrated in FIG. 15, a nut 121 of a ball screw mechanism 120 is supported so as to be rotatable via a bearing 130 with respect to a housing 110 that covers the periphery of a rack shaft 100. Elastic members 140 and plates 141 which are annular are provided in gaps between the bearing 130 and the housing 110 in an axial direction ZA of the rack shaft 100. The plates 141 prevent the elastic members 140 from contacting the housing 110. The bearing 130 is interposed between the elastic members 140, 140 in the rack shaft axial direction ZA. Consequently, the nut 121 is elastically supported in the rack shaft axial direction ZA by the elastic members 140, 140 via the bearing 130.
With such a configuration, when a force in the axial direction ZA is applied to the rack shaft 100 by a steering operation performed by the driver, the nut 121 and the bearing 130 initially move in the axial direction ZA together with the rack shaft 100 against an elastic force from the elastic members 140, and thereafter the nut 121 relatively rotates with respect to the rack shaft 100. Thus, the rack shaft 100 can easily move in the axial direction when a force in the axial direction ZA starts being applied to the rack shaft 100, which reduces the resistance against a steering operation at the initial stage. Therefore, a smooth steering feeling can be obtained.
A coned disc spring 142 such as that illustrated in FIG. 16A may be used as the elastic member 140. In the case where such a coned disc spring 142 is used as the elastic member 140, however, the coned disc spring 142 may be deformed into a closely contacting state illustrated in FIG. 16B when the bearing 130 is moved in the rack shaft axial direction ZA. The term “closely contacting state” refers to a state in which the length of the coned disc spring 142 in the axial direction is equivalent or close to the plate thickness. When the coned disc spring 142 is deformed into a closely contacting state, an excessive stress is applied to the coned disc spring 142. This may shorten the life of the coned disc spring 142.